The present invention relates to a method of producing on a polished facet of a diamond gemstone an information mark which is invisible to the naked eye, comprising using ultraviolet radiation or other radiation to irradiate the surface of the respective portion of the stone. "Invisible to the (naked) eye" means invisible to the (naked) eye of the expert or trained diamond grader. The information mark (which may be called an indicium) may be an identification mark for the diamond but need not identify a particular diamond and in general could give other information such as a quality or trade mark. Normally, the information mark will be a complex mark, not say a simple dot.
Such information marks are discussed in U.S. Pat. No. 5,410,125, from column 1, line 40 to column 2, line 2, but the technology of producing information marks on diamonds is also discussed in U.S. Pat. No. 4,392,476, U.S. Pat. No. 4,467,172, U.S. Pat. No. 5,149,938 and U.S. Pat. No. 5,334,280, and like technology is discussed in U.S. Pat. No. 4,912,298 in relation to spectacle lenses. In EP 0 567 129 A, U.S. Pat. No. 4,478,677, WO 90/03661, a paper by Geis et al in Appl. Phys. Lett. 55(22), pages 2295 to 2297, a paper by Harano et al in Advances in New Diamond Science and Technology, MYU Tokyo 1994, pages 497 to 500, a paper by Rothschild et al in J. Vac. Sci. Technol. B4(1), January/February 1986, pages 310 to 314, a paper by Rothschild et al in Proceedings of the SPIE, 1986, vol 633, Optical Microlithography V (1986), pages 51 to 57, a paper by Sercel et al. in Lasers & Optronics, September 1988, pages 69 to 72, and a paper by Sercel et al in Proceedings of the SPIE, vol 998, pages 76 to 83, there are discussions of similar technology.
There are relatively few published papers on the interaction of good quality gem diamond with intense pulsed ultraviolet lights such as those generated by excimer lasers. If diamond is discussed, most papers are concerned with polycrystalline diamond or "diamond like" films grown by chemical vapour deposition (CVD). The CVD material is often of poor optical quality and may contain significant amounts of carbon bonded in a graphite like (sp.sup.2) configuration rather than the pure diamond (sp.sup.3) configuration. Owing to the difference in the structures, these CVD materials are not likely to interact with ultraviolet radiation in the same way as good quality gem diamond material.
It is desirable to be able to produce the information mark in a manner which is not complex, which gives consistent results, which does not take too much time, which causes little pollution, and which gives no risk of damage to the diamond; it is also desirable to produce an even less visible mark which however with suitable magnification and viewing conditions is distinct and readable or identifiable.